As discussed in the above referenced co-pending applications one aspect of an EUV light source operating at ten to twenty thousand pulses of EUV light per second, or even higher, and using, e.g., a moving target, e.g., a mass limited droplet, is the ability to track the position and timing of the targets and their respective arrival at a desired plasma initiation site. This involves, e.g., determining a spot in 3D space which is imaged by an EUV light collector to an intermediate focus (IF), e.g., at an exit point for the EUV light from an EUV light generation chamber containing the collector and the desired initiation site. Also as discussed in the above referenced co-pending applications a droplet delivery system, including, e.g., a droplet generator and aiming system, needs to be aligned so that the droplets are projected through or fall through (in the case of a gravity feed) the spot that constitutes the desired plasma initiation site; corresponding to the focus of the collector and a small area around this focus, e.g., ±10 μm in which EUV generated from a laser produced plasma will still be adequately focused to the intermediate focus of the system, a so-called desired plasma initiation region around the desired plasma initiation site. Also required is to be able to fire the laser to have the drive laser beam intersect the target droplet at the desired plasma initiation site, i.e., when a droplet arrives exactly at the desired initiation site. It will be understood, as noted, that the desired initiation site may vary slightly from the precise focus of the collector, e.g., at a first focus of an elliptical collector mirror having a second focus comprising the intermediate focus of the light source system, e.g., by about 10 μm for, e.g., droplets of about 10 μm-40 μm in diameter and still be in focus enough for adequate collection. Therefore, a function of the tracking sub-system is not only to determine when to fire the laser(s) but at what selected plasma initiation site if not the desired plasma initiation site at the true focus, and the corrections necessary for the delivery system to, in the meantime, bring the target delivery to the desired plasma initiation site. The system may also determine that the target droplet is not on a track to arrive at the desired plasma initiation site and that, therefore there will not be any effective generation of EUV light that will arrive at the intermediate focus, and that therefore the laser should not be fired while the target droplets are returned to a proper target track to intersect the desired EUV plasma initiation site. Alternatively, laser firing could be allowed to continue even though adequate EUV light is not being collected while the target positioning is ongoing.
It will be understood that, while “desired plasma initiation site” as used herein is the focus of the collector, some area around the focus of the collector in which aiming the drive laser beam at a so-called selected plasma initiation site that is slightly off of the collector focus, can still be effective for generating an effective amount of EUV light at the intermediate focus (“the desired plasma initiation region”). “Selected plasma initiation sites” that are not on the collector focus, but within the desired plasma initiation region, having an acceptable distance error in both the x and y planes, as defined below, may occur. In the event that the laser will continue to be fired even if the selected plasma initiation site is outside the desired plasma initiation region, then selected plasma initiation sites may occur outside the desired plasma initiation region also.
Aspects of performing these functions have been discussed in the above referenced co-pending applications. Applicants herein propose certain improved apparatus and methods for accomplishing these functions.
Applicants have developed unique approaches to place the targets, e.g., individual Li droplet targets in the right position in 3D space, aiming a laser beam at a droplet position and firing the laser at the right moment in order to better enable operation of EUV LPP source, according to aspects of embodiments of the present invention. The irradiation of the target, e.g., a target droplet, heats the droplet sufficiently to cause the formation of a plasma through, e.g., evaporation/ablation and photons in the laser beam strip off electrons forming ions of evaporated target metal atoms in the plasma, and in this sense the target is ignited at a plasma initiation site, using the meaning of ignite or ignition to mean the subjecting of the target to intense heat and/or to heat up or excite, and generally meaning the formation of the plasma from the irradiated target due to the impartation of the heat (energy) from a drive laser beam intersecting the target and igniting the target to form a resultant plasma, that in turn produces EUV radiation. The use of the term ignition in the above referenced applications will be understood to have this meaning. Another meaning for ignition is the heating of a plasma to a temperature high enough to sustain nuclear fusion. While likely that such a temperature is attained in the plasma formation according to aspects of the present invention, which, however, involves none of the attempts to confine the plasma so formed according to aspects of the present invention sufficient to induce and/or sustain fusion, the conception of an ignition of a plasma according to aspects of an embodiment of the present invention has a similar meaning as used in the above referenced applications. In the present application the same concept is expressed by the term “plasma initiation” and “plasma initiation site,” meaning the irradiation of the target causes the plasma to form “plasma initiation” and this occurs or is desired to occur at some “plasma initiation site.”
Lithium for use as a target as discussed in above referenced co-pending applications likely will have at least some impurities in it. Even levels of impurities in the parts per million range, over time, can cause unwanted and damaging depositions within an LPP EUV chamber, e.g., on the collector optics and/or various chamber windows. These impurities, contained in an LPP target droplet of liquid lithium, after plasma initiation will be deposited, e.g., on the collector mirror. Since many of these impurities have much higher boiling temperatures than the, e.g., 400-500° C. proposed collector temperature, e.g., to evaporate the deposition of lithium itself, it is more difficult to remove these impurities from the collector using the previously suggested evaporation techniques. Applicants in the present application suggest a way of dealing with this problem in previously proposed LPP and/or DPP EUV chamber components, e.g., the optical components.
As discussed in prior co-pending applications referenced above, the collector needs to operate at an elevated temperature (e.g., at least at about a range of 400-500° C.), e.g., in order to evaporate Li from its reflective surface and maintain its reflectivity. Applicants propose in the present application apparatus and methods to maintain a stable and uniform temperature range across the optics of the collector over which its performance is able to meet required specifications, e.g., the avoidance of collector distortion due to maintenance of the elevated temperature.
Utilization of a solid state laser, e.g., a Nd:YAG laser to drive a LPP EUV source, with 1064 nm laser light is often doubled, has been known to employ doubled, tripled, etc. frequencies, e.g., to possibly achieve higher conversion efficiency at smaller wavelengths produced at the first harmonic generation (“FHG”) and second harmonic generation (“SHG”). This has been based on accessing a higher density plasma layer with the shorter wavelength higher harmonics, such that more source atoms are available for excitation and subsequent emission. In generating the higher laser harmonics, however, a large fraction (perhaps 30-50% for SHG and 80% for FHG to 266 nm) is lost because it is not converted in the nonlinear crystals.
Applicants have also developed, according to aspects of embodiments of the present invention ways to achieve higher conversion efficiency from laser energy converted to EUV radiation, and which allows extremely precise control of the initial density scale length, which will allow precision optimization of the laser deposition of energy into a target, e.g., a droplet, for improved conversion energy output ratios.
One of the problems in focusing optics for EUV LPP sources with Li or similar elements is a contamination and degradation of the optics due to contamination from Li or other elements. Applicants have developed according to aspects of embodiments of the present invention utilizations of grazing incidence optics or other EUV radiation collection optics for the improvement of conversion efficiency.
Also an issue in systems of the type of aspects of an embodiment of the present invention relates to the need for protecting optics other than the collector, e.g., windows and focusing optics, which may be combined, e.g., in introducing the drive laser beam into the EUV light source production chamber, which are addressed in the present application.